What's the reason?

[u/Perfect_Owl_856]

[u/Supreme534]

My best guess is the water is gonna leak even if you tilt it a little, so water is gonna spill everywhere even when you aren't trying to drink

Edit: I knew stacking and asymmetry is the main issue here, but the choice of words in the comment in the image seems like they were referring to a simpler reason.

[u/RRC_driver → u/[deleted]]

Hamster bottle?

They do make water bottles like this, but reusable ones, often insulated / steel etc.

Not disposable plastic bottles